Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity, and role in autophagy of MTMR8

Department of Pathology and Immunology, Washington University School of Medicine, St Louis, MO 63110, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 05/2012; 109(24):9539-44. DOI: 10.1073/pnas.1207021109
Source: PubMed


The myotubularins are a large family of inositol polyphosphate 3-phosphatases that, despite having common substrates, subsume unique functions in cells that are disparate. The myotubularin family consists of 16 different proteins, 9 members of which possess catalytic activity, dephosphorylating phosphatidylinositol 3-phosphate [PtdIns(3)P] and phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)] at the D-3 position. Seven members are inactive because they lack the conserved cysteine residue in the CX(5)R motif required for activity. We studied a subfamily of homologous myotubularins, including myotubularin-related protein 6 (MTMR6), MTMR7, and MTMR8, all of which dimerize with the catalytically inactive MTMR9. Complex formation between the active myotubularins and MTMR9 increases their catalytic activity and alters their substrate specificity, wherein the MTMR6/R9 complex prefers PtdIns(3,5)P(2) as substrate; the MTMR8/R9 complex prefers PtdIns(3)P. MTMR9 increased the enzymatic activity of MTMR6 toward PtdIns(3,5)P(2) by over 30-fold, and enhanced the activity toward PtdIns(3)P by only 2-fold. In contrast, MTMR9 increased the activity of MTMR8 by 1.4-fold and 4-fold toward PtdIns(3,5)P(2) and PtdIns(3)P, respectively. In cells, the MTMR6/R9 complex significantly increases the cellular levels of PtdIns(5)P, the product of PI(3,5)P(2) dephosphorylation, whereas the MTMR8/R9 complex reduces cellular PtdIns(3)P levels. Consequentially, the MTMR6/R9 complex serves to inhibit stress-induced apoptosis and the MTMR8/R9 complex inhibits autophagy.

Download full-text


Available from: Monita P Wilson, Oct 21, 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A large family of myotubularin (MTM) phosphatases dephosphorylates phosphatidylinositol 3-phosphate [PI(3)P] and PI(3,5)P2, which are known to play important roles in vesicular trafficking and autophagy. The family is composed of 16 members, and understanding their regulatory mechanisms is important to understand their functions and related genetic diseases. We prepared anti-MTMR6 monoclonal antibody and used it to study the regulatory mechanism of MTMR6. Endogenous MTMR6 was present in the cytoplasm and was condensed in the perinuclear region in a microtubule-dependent manner. MTMR6 preferentially interacts with GDP-bound Rab1B via the GRAM domain and partly overlapped with Rab1B in the pericentrosomal and peri-Golgi regions in NRK cells. Over-expression of GDP-bound Rab1B and the reduction of Rab1B disrupted the localization of MTMR6, suggesting that Rab1B regulates the localization of MTMR6. The reduction of MTMR6 accelerated the transport of VSV-G, in which Rab1B is involved. Furthermore, reduction of MTMR6 or Rab1B inhibited the formation of the tubular omegasome that is induced by over-expression of DFCP1 in autophagy. Our results indicate that the cellular localization of MTMR6 is regulated by Rab1B in the early secretory and autophagic pathways. We propose a new regulatory mechanism of MTM phosphatase by the small GTPase Rab1B.
    Preview · Article · Nov 2012 · Journal of Biological Chemistry
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum (ER) in skeletal muscle and is essential for calcium homeostasis. The mechanisms involved in SR remodeling and maintenance of SR subdomains are elusive. In this study, we identified myotubularin (MTM1), a phosphoinositide phosphatase mutated in X-linked centronuclear myopathy (XLCNM, or myotubular myopathy), as a key regulator of phosphatidylinositol 3-monophosphate (PtdIns3P) levels at the SR. MTM1 is predominantly located at the SR cisternae of the muscle triads and Mtm1 deficient mouse muscles and myoblasts from XLCNM patients exhibit abnormal SR/ER networks. In vivo modulation of MTM1 enzymatic activity in skeletal muscle using ectopic expression of wild-type or a dead-phosphatase MTM1 protein leads to differential SR remodeling. Active MTM1 is associated to flat membrane stacks, while dead-phosphatase MTM1 mutant promotes highly curved cubic membranes originating from the SR and enriched in PtdIns3P. Over-expression of a tandem FYVE domain with high affinity for PtdIns3P alters the shape of the SR cisternae at the triad. Our findings, supported by the parallel analysis of the Mtm1-null mouse and in vivo study, reveal a direct function of MTM1 enzymatic activity in SR remodeling and a key role for PtdIns3P in promoting SR membrane curvature in skeletal muscle. We propose that alteration in SR remodeling is a primary cause of X-linked centronuclear myopathy. The tight regulation of PtdIns3P on specific membrane subdomains may be a general mechanism to control membrane curvature.
    Full-text · Article · Feb 2013 · Journal of Cell Science
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Screening a complete mouse phosphatase lentiviral shRNA library using high-throughput sequencing revealed several phosphatases that regulate CD4 T-cell differentiation. We concentrated on two lipid phosphatases, the myotubularin-related protein (MTMR)9 and -7. Silencing MTMR9 by shRNA or siRNA resulted in enhanced T-helper (Th)1 differentiation and increased Th1 protein kinase B (PKB)/AKT phosphorylation while silencing MTMR7 caused increased Th2 and Th17 differentiation and increased AKT phosphorylation in these cells. Irradiated mice reconstituted with MTMR9 shRNA-transduced bone marrow cells had an elevated proportion of T-box transcription factor T-bet expressors among their CD4 T cells. After adoptive transfer of naïve cells from such reconstituted mice, immunization resulted in a greater proportion of T-box transcription factor T-bet-expressing cells. Thus, myotubularin-related proteins have a role in controlling in vitro and in vivo Th-cell differentiation, possibly through regulation of phosphatidylinositol [3,4,5]-trisphosphate activity.
    Preview · Article · Apr 2013 · Proceedings of the National Academy of Sciences
Show more

We use cookies to give you the best possible experience on ResearchGate. Read our cookies policy to learn more.